The present invention relates to an oxide superconductor wire and, more particularly, to an AC oxide superconductor wire with a large critical current and a reduced AC loss, a method of manufacturing the same, and an AC oxide superconductor wire cable.
As methods of reducing the AC loss in an oxide superconductor wire formed by embedding a large number of superconductor filaments in a metal matrix, decreasing the pitch of twist of each superconductor filament is known in addition to decreasing the diameter of each filament and increasing the specific resistance of the metal matrix.
The relationship between the twist pitch and the AC loss will be explained below.
FIGS. 1A and 1B are views for explaining the coupling current. In FIGS. 1A and 1B, reference numeral 1 denotes a superconductor filament; and 2, a metal matrix.
When a magnetic field is applied to a superconductor, a screening current flows in the form of a loop and prevents the magnetic flux from entering into the superconductor. The purpose of a multicore construction is to trap the magnetic flux inside and reflux a screening current in each superconductor filament, thereby increasing the stability. Even in such a multicore wire, as indicated by the arrows in FIG. 1A, screening currents can flow through the metal matrix between the superconductor filaments 1. These screening currents decay in the course of time due to the resistance of the metal matrix 2. If the length of the wire is increased, however, the inductance of the screening current loop increases, and this prolongs the decay time of the screening current. This state is called electromagnetic coupling. In thisk coupling state, a multicore wire behaves as if it were a single superconductor, and this eliminates the effect of a multicore construction.
To reduce this electromagnetic coupling, twisting the superconductor filaments 1 is effective. When the filaments are twisted, as shown in FIG. 1B, screening currents are refluxed in a region 1/2 of the twist pitch. Consequently, the inductance of the screening current loop decreases, and this accelerates the decay of the screening currents. When the decay time becomes shorter than the fluctuation period of the magnetic field, the coupling state as described above is prevented. A critical length L.sub.c by which the decay time of the screening current is shorter than the fluctuation period of the magnetic field is determined by a specific resistance .rho. of the metal matrix 2, a diameter d.sub.f and a critical current density J.sub.c of the superconductor filament 1, and a change rate dH/dt of an external magnetic field, and is represented by the following equation. EQU L.sub.c =2{(2.rho..multidot.d.sub.f .multidot.J.sub.c)/(.mu..sub.o .multidot.dH/dT)}.sup.1/2
That is, when the superconductor filament 1 is twisted at a pitch not larger than twice the critical length L.sub.c, it is possible to prevent a coupling current from flowing and reduce the AC loss.
Unfortunately, in an oxide superconductor wire it is very difficult to twist the superconductor filament 1 at a pitch not larger than twice the critical length L.sub.c. For example, when an oxide superconductor multicore tape wire (tape thickness 0.25 mm, tape width 3 mm, superconductor filament thickness 15 .mu.m, and critical current density J.sub.c =10.sup.4 A/cm.sup.2), as a general oxide superconductor wire, including silver with a specific resistance of 2.5.times.10.sup.-9 .OMEGA..multidot.m as the metal matrix 2, is applied with an AC magnetic field having change rate .mu..sub.o .multidot.dH/dt=1.2 T/sec in a direction perpendicular to the longitudinal direction of the wire and parallel to the wide surfaces of the wire, the critical length L.sub.c is 5 mm, so the twist pitch must be not larger than 10 mm which is twice as large as 5 mm. Additionally, in actual large-current AC cables or AC coils for generating a high magnetic field, the field conditions are more severe, and the critical length L.sub.c further decreases.
It is, however, extremely difficult to twist an oxide superconductor wire of a practical size at a pitch not larger than twice this small critical length L.sub.c. Therefore, it is conventionally reckoned that when it is not possible to twist an oxide superconductor wire at a pitch not larger than twice the critical length L.sub.c, the AC loss increases due to electromagnetic coupling of the superconductor filaments 1, so no AC loss reducing effect resulting from twisting can be expected.
An AC oxide superconductor cable manufactured by winding a plurality of AC oxide superconductor wires into a plurality of layers around a core member is also available. In this AC oxide superconductor cable, if the impedance (the sum of the resistance and the inductance) varies from one layer to another, a larger AC current flows in a layer with a lower impedance. This produces a localized current flow, which causes an anomalous increase of the AC loss. To make the impedances of the individual layers equal to each other, various methods by which only the inductance components of the layers are adjusted and equalized have been proposed. However, no practical method of equalizing the resistance components of the layers is known.
Assume that the inductances of the layers are made equal to each other and consequently the individual wires have the same ratio of the power supply current to the critical current and the layers have the same power supply current density. In this case, the farther the layer from the core, the stronger the self-magnetic field of the layer and the larger the AC loss (resistance component) resulting from magnetization, so the impedance of the layer increases for the same inductance component. Accordingly, a larger current flows into wires arranged in inner layer, having a relatively low impedance, and the AC loss in the inner layers increases. As a consequence, a large current flows this time in wires arranged in outer layers having a relatively low impedance, so a phenomenon of localized current flow occurs continuously. This brings about an abnormal increase of the AC loss in the whole cable.